Permanganate final rinse for metal coatings

ABSTRACT

A final rinse for conversion coated metal surfaces comprising an aqueous acidic solution of permanganate ions having a normality in the range from about 0.0001 to about 2.0 N is disclosed. The rinse of the present invention not only yields coatings having superior corrosion resistance but is also easily disposable without harmful effects on the environment.

nited States Patent 1191 Melotik H lMarch 13, 1973 [54] PERMANGANATE FINAL RINSE FOR 1,291.352 1/1919 Allen ..l48/6.l5 z ET L ATING 1,971,240 8/1934 Tosterud ..-..l48/6.2

Inventor: Donald J. Melotik, Dearborn, Mich.

Stauffer Chemical Company, New York,N.Y.

Filed: Feb. 16, 1971 App1.No.: 115,691

Assignee:

US. Cl. ....l48/6.l4 R, 148/614 A, 148/6.15 R, 148/6.15 Z, 148/62 Int. Cl ..C23f 7/08 Field 01' Search ..148/6.15 Z, 6.15 R, 6.14 A, 148/616, 6.2

References Cited UNITED STATES PATENTS 12/1918 Allen ..148/6.16

Primary Examiner-Ralph S. Kendall Atz0rneyWayne C. Jaeschke, John L. Callahan, Jr. and Martin Goldwasser [57] ABSTRACT A final rinse for conversion coated metal surfaces 9 Claims, N0 Drawings I BACKGROUND OF THE DISCLOSURE In the art of preparing metal surfaces to receive paint, it is widely known that the formation of various conversion coatings such as coatings of phosphates, mixed phosphate-oxides, chromates, oxalates and arsenates, on the surface of the metal substantially inlo creases the corrosion resistance and humidity resistance of the final painted surface. Likewise, the use of manganese phosphate coatings has been found to increase the lubricity of the metal surface. It is also well known that the effectiveness of these coatings is substantially increased by the application to the coated surface of a dilute solution of chromium in its various forms, i.e., chromic acid, hexavalent chromium and/or trivalent chromium; said solution optionally containing other ingredients such as phosphoric acid, a reducing agent for chromium such as formaldehyde or straight polyalcohols, wetting agents, pl-I adjusters and the like. Thus, in the commercial, high speed, production line processing of metal parts for painting, the predominant processes apply the phosphate or other conversion coating from an aqueous bath and, after water rinsing the formed chemical coating, it is subjected to the aqueous chromium rinse either by spraying or dipping. In many processes the chromium rinse step is then followed by an additional water rinse to remove uneven concentrations of the chromium on the surface of the metal. While all of these processes increase the corrosion resistance of the metal surfaces, they all have a very serious drawback, namely, the fact that the chromium contained in both the chromium and water rinses cannot effectively be removed from these solutions. Consequently, even after extensive efforts have been made to remove the chromium, some chromium will be carried over in the effluent of a metal processing plant into the environment, thereby creating a serious pollution problem.

It, therefore, is an object of the present invention to provide a composition and process for rinsing conversion coated metal surfaces to substantially increase the corrosion and humidity resistance of such surfaces, which composition and process does not pose a threat to the surrounding environment.

It is a further object of the present invention to provide a final rinse for conversion coated metals which rinse, while yielding coatings of superior corrosion resistance, can be easily and economically treated to yield relatively pure effluent from said process. Other objects will be apparent from the following description.

DETAILED DESCRIPTION OF THE DISCLOSURE It has nowibeen discovered that the objects of the present invention can be realized by employing a final rinse solution comprising an aqueous acidic solution of permanganate ions in lieu of the chromium type rinses in conventional high speed metal processing. More particularly, this invention provides a process which includes the conventional steps which are employed in high speed production line operation, namely, cleaning the metal surface, applying the phosphate or other conversion coating to the metal surface, and preferably water rinsing the metal surface, followed by a final rinse in the permanganate solution of the present invention. The permanganate rinse can also be followed by a water rinse to remove excess ions, however, this is not necessary. It is found especially desirable to employ a deionized water rinse prior to painting the surface by electrocoating techniques.

In accordance with the present invention, a conversion coated metal surface is rinsed with an aqueous acidic solution of permanganate ions. The term conversion coated metal surface is meant to designate surfaces of metals normally susceptible to corrosion such as iron, steel, zinc, aluminum, and the like, which surfaces have been coated with any of the well known conversion coatings such as the coatings of phosphates, mixed phosphate-oxides, chromates, oxalates, arsenates, and the like. These coatings are applied to the metal after it has been properly cleaned according to any of the well known degreasing and cleaning methods. Thus, the metal surface can be cleaned by solvent wiping, vapor degreasing, rinsing in an alkaline cleaning solution or combinations thereof. The cleaning step can alternatively be combined with the coating step by employing commercially available cleaningcoating solutions. Employing the alkaline rinse cleaning technique, itis customary to dip or spray the surface with the cleaning solution for a period of from about 10 seconds to about 10 minutes and then water rinse it for a like period of time. The metal surface can also be abraided, either by chemical or mechanical means, if deemed desirable. This abraiding step is most commonly accomplished by grit-blasting or acid pickling the metal surface and can be done prior to or after the cleaning step. Following the cleaning and/or abraiding steps, the metal surface is normally water rinsed to remove the cleaning and/or abraiding agents. The metal surface is then coated with the conversion coating by any of the known methods such as immersion, spraying, flooding, flowing or like techniques. As stated earlier, any of the well known conversion coating solutions can be used. However, the present invention is particularly adapted to phosphate coatings such as those containing zinc, calcium-zinc, iron and/or manganese phosphates. These preferred phosphate solutions are aqueous acidic solutions which can additionally contain various accelerators such as nitrite ions and chlorate ions, various well known wetting agents, particularly those of the anionic or non-ionic type and the like. Particularly preferred in the present invention are the zinc phosphate solutions. The phosphate solutions normally have a pH in the range from about 2 to about 6, with ranges from about 2 to about 4 being employed with zinc phosphate solutions. The conversion coating step is normally conducted by contacting the metal surface with the coating solution for a period of about 30 seconds to about 5 minutes at temperatures in the range of about to about 200F.

The conversion coating step can then be followed by a water rinse step. This rinseis optional, but it has been 0.000l to about 0.021 N and most preferably from about 0.00021 to about 0.00042. These solutions can be prepared by dissolving a sufficient amount of a suitable permanganate salt in water to yield the desired normality prior to use. Suitable permanganate salts for making the solutions of the present invention include the salts of the common alkali metals and alkaline earth metals, particularly potassium and sodium. Potassium permanganate is most preferred due to its availability. The pH of the solutions of the present invention must be adjusted to a value in the range from about 2 to about 6, preferably from about 2.8 to about 4.2, and most preferably 3.0 to 4.0. The pH is adjusted by adding suitable acids, particularly phosphoric acid to the permanganate solution. The rinse solutions of the present invention can additionally contain wetting agents and stabilizers which are compatible with the permanganate solution.

The permanganate rinsing of the present invention is accomplished by simply applying the rinse to the conversion coated metal surface by any of the well known rinsing methods such as spraying, dipping, brushing, flowing, flooding or the like. The metal surface should normally be in contact with the rinse solution for a period of from about 5 seconds to about 120 seconds. The temperature of the rinsing solution is not critical, however, it is normally in the range from about room temperature to about 180F.

After the permanganate rinse has been applied to the conversion coated surface, the surface can be water rinsed, however, this is not necessary. It has been found desirable to employ a deionized water rinse subsequent to the permanganate rinse and prior to painting the surface by any of the well known electrocoating techniques. The temperature of this water rinse can vary since it is not critical.

The metal surfaces, which have been treated by the process of the present invention can be painted wet or dried prior to painting. The drying can be accomplished by air drying, force drying or baking of the surfaces according to well known techniques. Painting of the metal surfaces is accomplished by spraying, dripping, electrocoating, electrostatic spraying, flowing or other well known methods.

A particular advantage of the present invention is the ease with which the effluent of the permanganate rinse can be purified. Thus, the pH of the rinse is raised to above 7 by addition of a base, and the permanganate is then reduced to manganese dioxide, which precipitates out, by suitable reducing agents, such as ferrous sulfate. Furthermore, dilute permanganate solutions when discharged to the stream, are not detrimental to the environment, even if untreated, since they are easily reduced by organic waste matter. In fact, permanganate has been used in the past as a water purifying agent. Likewise the reduction product, i.e., manganese dioxide, can be used in sewage treatment systems because of its beneficial sediment settling charac teristics.

EXAMPLE I Nine 4 X 12 inch cold rolled steel panels were cleaned and coated with an iron phosphate coating by spraying the panels for 1 minute with a combined cleaning and phosphatizing solution which is commercially available under the Trademark Metacote from Stauffer Chemical Company of Delaware, and then water rinsing for 1 minute. The panels were then final rinsed for 30 seconds in a bath of 0.00042 N potassium permanganate having a pH adjusted to 3.00 by addition of phosphoric acid. Three different final rinse methods, each employing three panels, were used as follows:

1. Potassium permanganate rinse alone;

2. Potassium permanganate rinse, followed by a deionized rinse; and

3. Potassium permanganate rinse followed by deionized water rinse and then baking at 250F. for 10 minutes.

All panels were dried and painted with Dupont 475-7435 black rubber primer and Dupont VB-20 black enamel. The panels were then scribed and subjected to a 5 percent salt spray for 168 and 336 hours. The average results are summarized in the following Table l. The rating system for these tests is determined by paint loss and corrosion advance from the scribe line measured in 32nds. of an inch as follows:

0 to 4 excellent to good 4 to 12 good to fair 12 to 28 fair to poor 28 poor to failure TAB LE l Rinse Scribe Creepage (in 32nds. of lnch) Sequence 168 Hours 336 Hours EXAMPLE II Nine 4 X 12 inch panels each of cold rolled steel and hot-dip galvanized steel were processed in like manner to the panels in Example I, except that they were first cleaned in an alkaline solution, water rinsed, and then coated with a zinc phosphate coating by spraying them in the same manner as Example I with a zinc phosphating solution commercially available under the Trademark Metacote X from Stauffer Chemical Company of Delaware. The average results are summarized in Table ll.

As a control, three 4 X 12 inch cold rolled steel panels that processed according to the procedure of Example I with the exception that a water rinse was substituted for the permanganate rinse. Upon salt spray testing for 336 hours, two of the panels showed creepage of slightly less than l6/32nds of an inch and the other panel showed marked failure.

Analysis of the above examples clearly demonstrates the superior results achieved by employing the rinse of the present invention.

What is claimed:

1. A process for increasing the corrosion resistance of a conversion coated metal surfaces which comprises rinsing a surface which has been coated with a conversion coating selected from the group consisting of phosphates, mixed phosphate-oxides, chromates, oxalates and arsenates, with an aqueous acidic nonchromium solution of permanganate ions, said solution having a normality in the range from about 00001 to about 2.0 N an a pH adjusted to a value in the range from about 2 to about 6.

2. The process of claim 1, wherein said permanganate ions are derived from potassium permanganate.

3. The process of claim 1, wherein said aqueous acid solution has normality in the range from about 0.00021 to about 0.00042 N and a pH in the range from about 3.0 to about 4.0.

4. The process of claim 1, wherein the pH is adjusted with phosphoric acid.

5. A process for treating metal surfaces to render them corrosion resistant which comprises:

a. cleaning said metal surface;

b. forming a conversion coating on said cleaned metal surface, said coating being selected from the group consisting of phosphates, mixed phosphateoxides, oxalate, arsenate and chromate; and

c. rinsing said conversion coated metal surface with an aqueous acidic non-chromium solution of permanganate ions having a normality in the range from about 0.0001 to about 2.0 N and a pH adjusted to the range from about 2.0 to about 6.0 by the addition of phosphoric acid.

6. The process of claim 5 wherein said conversion coating formed on the metal surface is a phosphate coating selected from the group consisting of zinc phosphate, calcium-zinc phosphate, iron phosphate and manganese phosphate.

7. The process of claim 5 wherein said conversion coating formed on the metal surface is a zinc phosphate coating.

8. The process of claim 5 containing the additional step of water rinsing said conversion coated metal surface prior to rinsing said surface with said permanganate ion solution.

9. The process of claim 5 additionally containing the step of water rinsing said conversion coated metal surface with deionized water after said surface hasbeen rinsed with said permanganate ion'solution. 

1. A process for increasing the corrosion resistance of a conversion coated metal surfaces which comprises rinsing a surface which has been coated with a conversion coating selected from the group consisting of phosphates, mixed phosphate-oxides, chromates, oxalates and arsenates, with an aqueous acidic non-chromium solution of permanganate ions, said solution having a normality in the range from about 0.0001 to about 2.0 N an a pH adjusted to a value in the range from about 2 to about
 6. 2. The process of claim 1, wherein said permanganate ions are derived from potassium permanganate.
 3. The process of claim 1, wherein said aqueous acid solution has normality in the range from about 0.00021 to about 0.00042 N and a pH in the range from about 3.0 to about 4.0.
 4. The process of claim 1, wherein the pH is adjusted with phosphoric acid.
 5. A process for treating metal surfaces to render them corrosion resistant which comprises: a. cleaning said metal surface; b. forming a conversion coating on said cleaned metal surface, said coating being selected from the group consisting of phosphates, mixed phosphate-oxides, oxalate, arsenate and chromate; and c. rinsing said conversion coated metal surface with an aqueous acidic non-chromium solution of permanganate ions having a normality in the range from about 0.0001 to about 2.0 N and a pH adjusted to the range from about 2.0 to about 6.0 by the addition of phosphoric acid.
 6. The process of claim 5 wherein said conversion coating formed on the metal surface is a phosphate coating selected from the group consisting of zinc phosphate, calcium-zinc phosphate, iron phosphate and manganese phosphate.
 7. The process of claim 5 wherein said conversion coating formed on the metal surface is a zinc phosphate coating.
 8. The process of claim 5 containing the additional step of water rinsing said conversion coated metal surface prior to rinsing said surface with said permanganate ion solution. 